Numerous compounds are known which release photographically useful groups in accordance with the image density during development. Hydroquinone derivatives (e.g., DIR hydroquinone) are known as compounds which release development restrainers in correspondence to image
density during development. Exemplary compounds known as DIR hydroquinones include those described in U.S. Pat. Nos. 3,379,529 and 3,620,746 and Japanese Patent Application (OPI) No. 129,536/74. (The term "OPI" as used herein refers to a "published unexamined Japanese patent application.")
As in known from the above-cited references, DIR hydroquinone is used to obtain the so-called DIR effects, such as tone control, finer-grain images, improved sharpness and better color reproduction and the like. Conventional DIR hydroquinones show a certain degree of these effects, but further improvement has been required.
In particular, known DIR hydroquinones have the following disadvantage:
As the restrainer released during development becomes diffused from the sensitive material into the developing solution, it is accumulated in the solution. As a result, the solution shows restraining effects. If a large quantity of the sensitive material is treated on a continuous basis, as in the method commonly employed on a commercial scale, it is difficult to always obtain a constant color tone. Thus, the contamination of the development solution with the restrainer released by DIR hydroquinone is a serious problem.
In order to solve this problem, various measures have been taken as a matter of convenience, but they all have some disadvantages and no fundamental solution has not yet been known. For example, the following methods are known:
(1) Restriction of the amount of DIR hydroquinone used for the development;
(2) Frequent change of the developer solution for a new supply; and
(3) Incorporation in the sensitive material of a microcrystal emulsion layer free from any substantial photographic sensitivity as a scavenger for the restrainer released from the sensitive layer.
However, these methods all ave disadvantages such as the limitation of photographic improvement and a large increase in cost.